click on colours/others to merge with and then tell what colour you would like to merge with. It will automatically merge to a higher colour/others.
Plant square, 64 square, Star square and Lemonlime are the hardest. Big update. If clicking other things that should say what square is it doesn't say that press yellow to refresh. I would advise to write the merges on paper incase you don't remember them. Hint star + yellow is not sun. Its yellow + orange.